This invention relates to composite materials for hard tissue replacement, and to methods for producing such materials. More particularly, the invention relates to collagen-containing composites, substrates coated with calcium, phosphate-containing components and methods for producing same. The collagen-containing composite materials can be prepared by precipitation of calcium, phosphate-containing materials, such as calcium apatite materials, in a collagen slurry. The coated calcium, phosphate-containing substrates can be prepared by contacting an uncoated substrate in a supersaturated or oversaturated solution of coating materials, such as calcium apatites, to coat the surface of the substrate.
The composition of hard tissue, such as natural bone, comprises collagen and inorganic calcium phosphate, particularly biological apatite. Bone contains about 60% to 75% by weight of biological apatite and tooth has more than 98% by weight of biological apatite. Biological apatite is a naturally occurring calcium apatite-type material which is formed in the body by precipitation from body fluids at body conditions. This biological apatite has a structure which is similar to pure hydroxyapatite, but contains some substitute ions for the calcium, phosphate and hydroxyl ions. Strictly speaking, synthetically produced precipitated hydroxyapatite is more similar to biological apatite than are the hydroxyapatite ceramics.
Synthetically produced precipitated hydroxyapatite is a very fine powder. It has limited application as a hard tissue implant material because it is somewhat difficult to manipulate.
Recently, premolded collagen incorporated with a granule form of hydroxyapatite ceramic (HA) have been commercialized. In such collagen-HA composites, the collagen and hydroxyapatite ceramics form a completely heterogeneous mixture. In effect, the collagen serves as the binder system for the granules of the hydroxyapatite ceramics. Such heterogenous composite materials are completely different from the natural bone. It would be advantageous to provide a hard tissue replacement material which has an increased similarity to natural bone.
In the last decade or so, many calcium phosphate-containing ceramics and glasses have been prepared in granule form or block form for hard tissue substitute materials. Among these are hydroxyapatite ceramics, tricalcium phosphate ceramics and calcium, phosphate-containing bioglasses. Clinical studies have confirmed that hydroxyapatite ceramics and such bioglasses are very good biocompatible materials for use as artificial hard tissue implant materials. After implantation, hydroxyapatite ceramics and calcium, phosphate-containing bioglasses bond very strongly with bone.
The hydroxyapatite ceramics, as well as the bioglasses, are somewhat different from biological apatite. In general, hydroxyapatite ceramics and bioglasses after implantation take a certain amount of time to adjust to the body's environment. Biological apatite from new bone starts to grow and bind to the surface of these implant materials. For example, calcium, phosphate-containing bioglass, after soaking in body fluid, takes several weeks to form a precipitated hydroxyapatite surface layer. The time for the formation of a hydroxyapatite layer depends strongly on the composition of the bioglass. Clinical studies have showed that the bonding strength between the bioglass and the bone during the initial healing stage also depends on the rate of formation of this hydroxyapatite surface layer. In general, those bioglasses with a fast rate of formation of precipitated surface hydroxyapatite also show a strong bonding with bone. Even though untreated hydroxyapatite ceramics and bioglasses have excellent biocompatibility, it would be advantageous to provide additional biocompatibility and to provide hard tissue replacement materials which shorten the healing time.